Coffee extract, to the industry, is considered to be an aqueous solution of soluble solids extracted from the coffee bean. It is an intermediate stage product in the manufacture of freeze-dried and spray-dried (regular instant) coffees. It is also used directly in flavorings and in certain vending operations.
Until and even after World War II, coffee extract was batch-brewed from roasted and ground coffee and then dried. The flavor was poor, in part because of technical problems in dehydration. The dried product was hygroscopic and caked readily while reconstituting poorly. However, it was welcomed in the Army C Ration which gave a boost to post-war product acceptance.
Later, it was found that the addition of malto-dextrin (corn syrup) improved the flavor by aiding the dehydration process, reduced hygroscopiscity (caking), greatly aided reconstitution and made measurement more convenient.
Intense post-war competition as companies crowded into this rapidly growing market called for selling a "pure instant coffee". The expended coffee grounds became the obvious source of solubles to replace malto-dextrin. There were three potential approaches:
1. The first was enzymatic hydrolyzation. It had many potential advantages. However, this approach was abandoned because the yield of solubles obtainable was too low to be commercially feasible.
2. Acid hydrolysis; this was highly effective but required salt removal when the pH was restored to acceptable levels for the beverage. It was represented by U.S. Pat. Nos. 2,573,405 and 2,687,399 but was rejected by the industry.
3. High temperature, high steam-pressure hydrolysis; was first used commercially in 1950 and soon became the system of choice worldwide in the manufacture of instant coffee. Morganthaler was issued U.S. Pat. No. 2,573,405 in 1943 covering this process.
This process utilizes high temperature (up to 175.degree. C.), high steam-pressure hydrolysis. The residence time is long, up to 2-3 hours.
The process and its operating problems and limitations are described in detail in "Coffee Processing", Volume 2, Sivetz & Foote, 1963 and in the updated version, "Coffee Technology", Sivetz & Desrosier, 1979, both by AVI Publishing.
Using the Morganthaler process the industry's standard yield of soluble solids (dry weight basis) based on the weight of the green coffee used grew to 40%. This varies, of course, depending on the raw material source, degree of roast and other operating conditions.
It is recognized in the industry that an increase of only 5.degree. C. above the patent's 175.degree. C. can degrade the substrate in only "15-30 minutes together with the extract thereby produced and results in an unacceptable product". Increased residence time beyond the 2-3 hours standard in the industry results in an "over extracted", undrinkable beverage. (References: "Coffee Processing", Volume 2 and "Coffee Technology" referred to above).
Besides the limitation to higher yields described above, the liquid coffee extract produced by this system has deficiencies of gel formation (undesirable when the concentrate is used for liquid coffee vending) and viscosity which limits evaporative concentration which would permit optimal utilization of the dehydrators. Enzymes are used in reducing these problems. (See U.S. Pat. No. 2,801,920, 1957).
In 1942 there were issued to John L. Kellogg U.S. Pat. Nos. 2,282,138 and 2,282,139. The concepts patented, including the use of an enzyme (diastase) and "relatively high steam pressure" as a pre-treatment of the ground roasted coffee to increase the yield of solubles were major advances in soluble coffee technology.
Kellogg called for using a converting enzyme diastase ("preferably Taka-diastase", Takemine U.S. Pat. No. 1,391,219, 1921) at temperatures of 125.degree.-135.degree. F. after pre-treating the coffee at 15 psi for one hour "to soften and loosen the fibres". He refers to obtaining and preserving more of the "delicate aroma" of coffee and to obtaining a "greatly enhanced yield of extractives". Unfortunately, Kellogg gave no figures to substantiate his claims. Others, trying this system, found the yield too low to be commercially practical, particularly in comparison to the yield possible using the Morganthaler process.
Kellogg was greatly interested in aroma preservation and protection from bacterial degradation which he believed his system, using taka-diastase, would provide. Although his enzymatic hydrolysis system appeared to have merit, his choice of diastase as the converting enzyme is questionable. Diastase solublizes starch, primarily to dextrose. In 1942 little was known about the chemical composition of roasted coffee. "All About Coffee", William H. Ukers, Second Edition, 1935 published by Tea & Coffee Trade Journal (the most advanced technical book at that time) acknowledges this fact.
It was not until 1985 that a substantial analysis of coffee chemistry was available. "Coffee: Botany, Biochemistry and Production of Beans and Beverage", edited by M. N. Clifford and K. C. Willson, published by Croom Held, London, reported the work of many researchers. From this it is possible to obtain an approximate chemical analysis of roasted coffee. Surprisingly, the starch content (which is the constituent which diastase solubilizes) is less than 1/2% according to the average of the findings reported.
The above work also indicated that cellulose makes up about 50% of the weight of roasted coffee and would, therefore, be the most important constituent to solubilize. However, "Coffee Technology", Sivetz & Descrosier, 1979, states on page 369 under the heading "Useless Techniques", "the use of enzymes to solubilize cellulose portions of green or roast coffee is impractical".